Fix complete_elements in DADict/DASet and add cancel_add_or_edit to DADict

[jpagh]

I'm sorry that this PR does more than one thing.

First, it fixes the complete_elements methods of DADict and DASet to return a new instance of their class with the complete_elements, rather than returning a generic dict or set. This matches the behavior of the method in a DAList and gives access to all of the DA methods included expected string output.

Second, it adds the cancel_add_or_edit method to DADict. This is very similar to the DAList version it is based on, but handles deleting the new_item_name attribute and uses popitem to remove the last item from the dict.

Third, it fixes the logic in DAList's cancel_add_or_edit method to only pop the last item if self.complete_elements().number() is less than self.number_gathered() rather than only if they are unequal. This is just a safety net for the (maybe not even possible) edge case where the length of complete_elements is somehow bigger than number_gathered, in which case you can't know if you want to pop the last element off of the DAList. I wrote the DADict version using this same logic. In all normal cases this works as expected and the same as with !=.